A new method to measure the inelastic pp cross section has been developed based on the assumption that pile-up events are randomly distributed according to a Poisson probability. This method relies only on the accurate knowledge of the CMS tracking system and it does not depend upon a specific Monte Carlo simulation. The value of the cross section for events with at least 2 charged particles with $|\eta|<2.4$ and $p_\perp > 200 $ MeV is determined to be $ \sigma_{2-trks} = 58.7 \pm 2.0 \; (Syst) \pm 2.4 \; (Lum) \; {\rm mb} $. Using model-dependent extrapolations, the value for the total inelastic $pp$ cross section is determined to be $ \sigma_{inel}(pp) = 68.0 \pm 2.0 \; (Syst) \pm 2.4 \; (Lum) \; \pm 4 (Ext.) \; {\rm mb} $.
Figure | Abbreviated Caption |
---|---|
Fraction of generated events with a given vertex track multiplicity accepted by the selection criteria $NDOF \ge 0.5$ and with either 2 (empty circles) or 3 (solid circles) reconstructed tracks. The plot shows this fraction evaluated with 2 different versions of PYTHIA. | |
Comparison data - reweighted Monte Carlo. Each plot shows, as function of luminosity, the number of events with a given value of vertices, from 1 to 11. The first 9 plots contain the data used in this analysis. | |
Data and Monte Carlo distribution in each of the 13 luminosity bins, from 0.05~to~0.7~$\cdot 10^{30}\; {\rm cm^{-2} \; s^{-1}}$. | |
Fraction of events with n pile-up events as a function of luminosity. The dotted line is a Poisson fit. | |
$\chi^2$/NDOF (top) and $\sigma(pp)$ values (bottom) for each of the fit of Figure~\ref{fig:final}. The dashed line is the result of the fit to the 9 $\sigma(pp)$ values. The fits are for events with at least 2 charged particles. |
Comparison between the CMS results and several Monte Carlo models for events with at least 2, 3 and 4 charged particles in $abs(\eta)<2.4$ and $p_t > $ 200 MeV. Systematic uncertainties shown by the inner red error bars and luminosity uncertainty by the outer black bars. Monte Carlo predictions have a common uncertainty of $~1 $ mb |
Compilation of total inelastic $pp$ and $p\bar{p}$ cross section values. CMS analysis uncertainty is shown in dark blue while the model-dependent extrapolation is shown in light blue. |
Generator level $\xi$ distribution of several MC models (PYTHIA, PHOJET, SIBYLL, QGSJET, EPOS) for all events and for events with at least 2 charged particles, each with $p_t > 200$ MeV in the range $abs(\eta)< 2.4$. |
Figure | Description |
---|---|
Number of vertices as a function of the bunch instantaneous luminosity. | |
Number of vertices during the 2010 run period | |
The x-axis of the plot is the distance between two vertices while on the y axis it shows their combined track multiplicity. This plot shows that vertices that are closer to each other more than ∼ 0.6mm are merged together. | |
Uncorrected pile-up distribution. The dashed lines are a fit using a Poisson distribu- tion, see eq.1. | |
Monte Carlo events fitted with a Poisson function for each value of pile-up events | |
χ2 (top) and σ values (bottom) for each of the fit of Figure 17. The dashed line is the result of the fit to the 9 σ values. The generator value of σinel = 56.2 mb is well reproduced by the measurement. |
Fraction of secondary vertices with and without the requirement on the transverse position of the vertex to be within ±0.06 mm from the beam line. |